how the hell did that slip by?
(for those who are not aware, there’s a single frame in this gif where Tails’ legs turn brown)
Well, the short answer is time, most cartoons, especially on TV, outsource a lot of their animation and colouring overseas to Japanese and Korean studios because they’re cheap. Mistakes mean re-shoots, and re-shoots take time and time is money. So they probably did notice it, decided it wasn’t worth the time and effort to fix (you also need to remember that the cels are physically painted, so it’s not like you can just use the fill tool or ctrl-z)
In fact, I recently had to clean up a cel I bought because the paint from the cel that had been placed on top of it had adhered to it (fortunately not the reverse where that cel’s paint would have been). So I had to give it a go-over with a few earbuds and some window cleaner. I’m no expert on how studios work but I imagine if they did mess up with a cel they’d just start over from scratch. Again, time and money.
Remember, DiC stands for ‘Do it Cheaply’.